Name

Library

Synopsis

Return values

Otherwise, a value of -1 is returned and errno is set to indicate the error.

Detailed description

Limitation :Here the difference between two fd’s passed  should not be greater than 8 , if the difference between 
two fd’s is greater than 8 then behaviour of fcntl system call is undefined.
An example call would be:

                  
fcntl (5, F_DUPFL, 10);
Here difference between two fd’s is  (10 - 5) = 5 which is less than 8

                  

The flags for the F_GETFL and F_SETFL flags are as follows:

Several commands are available for doing advisory file locking; they all operate on the following structure:

struct flock {
        off_t   l_start;        /* starting offset */
        off_t   l_len;          /* len = 0 means until end of file */
        pid_t   l_pid;          /* lock owner */
        short   l_type;         /* lock type: read/write, etc. */
        short   l_whence;       /* type of l_start */
};

         

When a shared lock has been set on a segment of a file, other processes can set shared locks on that segment or a portion of it. A shared lock prevents any other process from setting an exclusive lock on any portion of the protected area. A request for a shared lock fails if the file descriptor was not opened with read access.

An exclusive lock prevents any other process from setting a shared lock or an exclusive lock on any portion of the protected area. A request for an exclusive lock fails if the file was not opened with write access.

The value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END to indicate that the relative offset, l_start bytes, will be measured from the start of the file, current position, or end of the file, respectively. The value of l_len is the number of consecutive bytes to be locked. If l_len is negative, l_start means end edge of the region. The l_pid field is only used with F_GETLK to return the process ID of the process holding a blocking lock. After a successful F_GETLK request, the value of l_whence is SEEK_SET.

Locks may start and extend beyond the current end of a file, but may not start or extend before the beginning of the file. A lock is set to extend to the largest possible value of the file offset for that file if l_len is set to zero. If l_whence and l_start point to the beginning of the file, and l_len is zero, the entire file is locked. If an application wishes only to do entire file locking, the flock system call is much more efficient.

There is at most one type of lock set for each byte in the file. Before a successful return from an F_SETLK or an F_SETLKW request when the calling process has previously existing locks on bytes in the region specified by the request, the previous lock type for each byte in the specified region is replaced by the new lock type. As specified above under the descriptions of shared locks and exclusive locks, an F_SETLK or an F_SETLKW request fails or blocks respectively when another process has existing locks on bytes in the specified region and the type of any of those locks conflicts with the type specified in the request.

This interface follows the completely stupid semantics of System V and -p1003.1-88 that require that all locks associated with a file for a given process are removed when any file descriptor for that file is closed by that process. This semantic means that applications must be aware of any files that a subroutine library may access. For example if an application for updating the password file locks the password file database while making the update, and then calls getpwnam to retrieve a record, the lock will be lost because getpwnam opens, reads, and closes the password database. The database close will release all locks that the process has associated with the database, even if the library routine never requested a lock on the database. Another minor semantic problem with this interface is that locks are not inherited by a child process created using the fork system call. The flock interface has much more rational last close semantics and allows locks to be inherited by child processes. The flock system call is recommended for applications that want to ensure the integrity of their locks when using library routines or wish to pass locks to their children.

The fcntl, flock, and lockf locks are compatible. Processes using different locking interfaces can cooperate over the same file safely. However, only one of such interfaces should be used within the same process. If a file is locked by a process through flock, any record within the file will be seen as locked from the viewpoint of another process using fcntl or lockf, and vice versa. Note that fcntl (F_GETLK);
returns -1 in l_pid if the process holding a blocking lock previously locked the file descriptor by flock.

All locks associated with a file for a given process are removed when the process terminates.

All locks obtained before a call to execve remain in effect until the new program releases them. If the new program does not know about the locks, they will not be released until the program exits.

A potential for deadlock occurs if a process controlling a locked region is put to sleep by attempting to lock the locked region of another process. This implementation detects that sleeping until a locked region is unlocked would cause a deadlock and fails with an EDEADLK error.

Examples

/**
  * Detailed description : Sample usafe of fcntl system call
**/
#include <stdio.h>
#include <fcntl.h>
int main()
{
  int fd  = 0;
  int flags = 0;
  fd = open("Example.txt " , O_CREAT | O_RDWR  , 0666);
  if(fd < 0 ) 
  {
     printf("Failed to open file Example.txt\n");
     return -1;
  }
  if( (flags = fcntl(fd , F_GETFL) ) < 0 )  
  {
     printf("Fcntl system call failed \n");
     return -1;
  }
  printf("Flags of the file %o \n" , flags);
  return 0;
}

         

Output

Flags of the file 2

         

Errors

In addition, if fd refers to a descriptor open on a terminal device (as opposed to a descriptor open on a socket), a cmd of F_SETOWN can fail for the same reasons as in tcsetpgrp and a cmd of F_GETOWN for the reasons as stated in tcgetpgrp.

See also

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